1. Field of the Invention
The present invention relates generally to fluid bearings for shafts rotating at high speeds. More particularly, the invention concerns gas bearings having special shaft geometries for generating dynamic pressures within the interior of the bearing to precisely control the gap between the shaft and journal as the shaft rotates.
2. Discussion of the Prior Art
Gas bearings have been in use for many years in applications wherein a shaft rotates at very high speeds within a stationary journal. The well known advantages of these devices are very low frictional losses and lack of a complex oil lubricating system, usually comprising an oil reservoir, oil pump, filters, coolers, pressure gauges, valves and related hardware. The equally well known disadvantages of the devices are stringent requirements on manufacturing dimensional tolerances and surface finishes, with attendant high production costs. Further, such devices are plagued by the possibility of catastrophic damage due to the intrusion into the device of extremely small foreign objects, such as dust particles. Another disadvantage is the possibility of hydrodynamic instabilities under certain operating conditions.
In all fluid supported bearings, the hydrodynamic action which generates dynamic pressures within the space between the shaft and the journal is caused by the motion of a viscous fluid (whether oil or gas) which is forced to flow in a gap of varying width by the relative motion of the shaft within the journal. In the simplest case of a cylindrical shaft rotating in a cylindrical journal, the variation in the width of the gap is the consequence of an eccentricity of the shaft in the journal, such as may be caused by the weight of the shaft or other externally applied forces. This simplest bearing is stable under certain conditions, but becomes unstable when the combination of applied forces and rotational speed exceeds a certain value. Furthermore, the load-carrying capacity of the simple gas bearing is not very high.
Considerable ingenuity has been exercised in introducing modifications to the cylindrical surface of the journal to achieve a more effective variation of gap width for smaller values of eccentricity. These prior art endeavors have resulted in a variety of shapes being proposed, as for example, three-lobe bearings, lemon-shaped bearings, half lemon-shaped bearings, and displaced bearings. However, in the case of gas bearings, the problem is complicated by the low viscosity of the gas, typically on the order of one-thousandth of that of oil. As is well known, the clearance varies with the cube root of the viscosity for equivalent bearing stiffness. Therefore, gaps, or clearances, are required on the order of one-tenth or less than that required for oil bearings. Additionally, the requirement for such very close clearances creates a serious problem when temperature differences cause a differential expansion of shaft and journal. For example, if the shaft is colder than the journal, the clearance may become unacceptably large, with consequent instability. In the opposite situation, the shaft may grow larger than the journal and the bearing can seize.
Solutions to the aforementioned problems have been attempted by forming the journal out of a series of flexible, elastic elements, or foils, which conform to the dimensions of the shaft and at the same time provide by their overlap and/or deflection the required wedge-shaped variation of gap width. Exemplary of such an approach are the devices disclosed in U.S. Pat. No. 3,795,427, issued to Licht et al., and references therein.
Another class of prior art gas bearings has been suggested wherein the journal retains a cylindrical shape and the gap-varying geometry is transferred to the shaft. In this class of devices, the shaft is modified by providing recessed areas of various shapes, such as chevrons, herring bones, helices, and the like. However, these types of bearings suffer from disadvantages similar to those previously discussed, since the operating temperature difference between shaft and journal can cause either instability or seizure.
The fluid bearing of the present invention belongs to the general class described in the preceding paragraph, since in the preferred form of the invention the journal is smooth and round, and the shaft carries the gap-controlling geometry. However, as will become readily apparent from the discussion which follows, the shape and size of the recesses provided in the shaft surface differ markedly from common practice and are such that stability is enforced with allowable clearances much greater than that found in comparable prior art gas bearings. Consequently, the manufacturing tolerances are correspondingly relaxed, and the bearing can function properly over a much larger temperature range without difficulty. Further, the adverse effects of dust in the gas are minimized. In addition, a simple, automatic method is provided for adjusting the clearance in case the temperature variation exceeds that which can be accommodated in the basic journal.